Transdermal drug delivery is a diverse emerging field that will require a broad range of microneedle structures and materials. Hollow microneedles are a particularly significant aspect of this technology that present fabrication challenges for designers of microneedle arrays.
Patches containing arrays of microneedles are being investigated by many entities. It is known that the skin to be penetrated by microneedles for the delivery of a drug includes many layers that may vary in thickness. The outermost layer of the skin is the stratum corneum, which is usually between 10 and 20 microns (10-20 μm) thick. This layer presents the greatest barrier to transdermal flux of drugs or other molecules into the body. Below the stratum corneum is the epidermal layer which may be between 50 and 100 μm thick. Immediately below the epidermis is the dermis which is between 1 and 3 mm thick. A more detailed description of the skin layers and the desired range of dimensions of microneedle arrays is found in U.S. Pat. No. 7,211,062 B2 at columns 3 and 4, the disclosure of which is incorporated herein by reference in its entirety.
Transdermal patches of the prior art typically are manufactured from polymers that exhibit high rheology, even at elevated temperatures. The challenge in manufacturing a patch containing arrays of microneedles lies in accurately forming these high rheology polymers into very small, precise microneedle structures of high-aspect ratio. It is particularly difficult to use such high rheology polymers to form microneedle arrays having fine channels running through their length that allow a drug to pass through the microneedle to be delivered to the skin of a user. There are also concerns regarding potential polymer interactions and biocompatibility through clinical testing requirements.